The present invention relates in general to visual inspection apparatus and, in particular, to image detection technology for use with such apparatus. More particularly but not limited to, the invention relates to image pickup devices for use in detecting a two-dimensional (2D) image of an object to be sensed through the main scanning (H scanning) and subsidiary scanning (V scanning) operations.
One typical approach to detection of a 2D image using a linear image sensor is to read image data out of the linear image sensor while simultaneously permitting a projection image of an object being sensed on the image formation plane to move in a direction perpendicular to the linear image sensor, wherein the internal scanning (H-scanning) gets started every time the projected object image moves a distance corresponding to a single pixel (V-scanning). This H-scan distance may be the size of one pixel in the V direction.
For detection of a movement amount (V-scanning) of the object image in this case, a scale for use in detecting the coordinate system of a workpiece mount table is employable, by way of example.
Currently available scale and its associative detection scheme for detecting the movement amount or coordinates (position) of an object being sensed may include a variety of types of techniques based on various principles, such as the transmitted light detection scheme, diffraction light interference scheme and the like. In any one of these approaches, a graduation detection signal is of a sinusoidal waveform having two phasesxe2x80x94phase xe2x80x9cAxe2x80x9d and phase xe2x80x9cBxe2x80x9dxe2x80x94with the phase difference of 90xc2x0, the sine-wave signal being wave-shaped into a pulse signal for use as a coordinate signal.
In addition, in order to obtain a higher degree of resolution than the graduation of scale, a certain scheme is widely employed which employs a divider circuit for dividing the period of a graduation detection signal into several tens of segments or more.
The term xe2x80x9cscale resolutionxe2x80x9d as will be used in the description below refers to the resulting degree of resolution after execution of the period division processing, rather than the resolution of the scale per se.
It is also noted that the term xe2x80x9cscale detection pulsexe2x80x9d as used herein refers to a pulse signal obtainable after execution of the division processing in cases where the period is divided.
Conventionally, setup of a pixel size along the movement (in the V direction) of the object to be sensed is such that the scale detection pulse is such that the scale detection pulse is frequency-divided at a predefined frequency division ratio to provide an internal scan start signal of a linear image sensor used. This is because the linear image sensor is internally scanned at equal distances or intervals each equivalent to the pixel size.
Note that one prior known technique for variable control of the accumulation time length in order to control the detection sensitivity of a linear image sensor is disclosed in JP-A-62-225081.
Another technique is disclosed in JP-A-6-133209, which is to deal with CCD shift clocks independently of each other at the upper and lower ends of a known time delay and integration (TDI) image sensor in the case where an image within a pickup view filed is different in movement speed or rate between the upper and lower ends, thereby equalizing them to respective image movement rate values.
In visual inspection apparatus for use in inspecting the quality of a workpiece under manufacture including a semiconductor wafer or membrane, where comparative inspection is carried out with respect to repetitive patterns, it is desirable that two images under comparison be sensed or picked up in a way such that the pattern period or cycle is an integer multiple of the pixel size to thereby ensure that the resultant pattern-to-pixel positional relation is kept constant in any events.
More specifically, as shown in FIG. 5, when comparing patterns 51a and 51b to each other, it is desirable to compare image information items detected at pixels 61 and 62 that are identical to each other in pattern-to-pixel positional relation; to this end, the distance of such comparative patterns may be an integer multiple of the pixel size. Unless the comparative pattern distance is an integer multiple of the pixel size, detection might be done at a position such as a pixel 63 with respect to the pattern 51b, for example. In such a case, the resultant image information could contain some errors even when no differences are present in the patterns per se, which would result in correct comparison result being no longer expectable.
Conventionally, in an image pickup device of the type which detects a 2D image by use of a linear image sensor while moving (V scanning) an object to be sensed, the pixel size in the V direction is set by starting the internal scanning of such linear image sensor every time an object being sensed moves or travels a fixed distance, i.e. one pixel size.
In cases where a detection pulse of an object V-coordinate detection scale is utilized as the internal scan start signal of the linear image sensor, the unit of a minimal variable amount of such image sensor scan start signal is the resolution obtainable from the scale (either the resolution of graduation or a resolution as divided therefrom by signal processing), which is a digital value.
Accordingly, when a given distance segment L is divided by a pixel number M, the minimal variable amount relative to a distance corresponding to M pixels might be equal to the xe2x80x9cM multiplication of the scale resolution,xe2x80x9d which results in difficulty in any precise adjustment. In other words, when the distance L=Kxc2x7lu (where K is a given integer, and lu is the scale resolution), the pixel number capable of equally dividing the distance L is limited only to those divisors of the integer K.
For example, suppose that a given distance L=525 lu is to be divided into fifty pixels. If the pixel size=10 lu, then the result is (52 pixels+reminder 5 lu). In this respect, when the pixel size is enlarged or expanded by a minimal variable amount 1 lu obtaining the value 11 lu, the resultant pixel number is equal to (47 pixels+reminder 8 lu). Obviously, this value is far less than the target value of fifty pixels, which in turn makes it impossible to set at the desired pixel number.
In the case of such image pickup device using a TDI image sensor, it is the basic condition for guaranteeing achievement of normal operations of the TDI image sensor to let the distance on an object plane, which distance corresponds to the total TDI stage number, be exactly an integer times the pixel size in order to ensure that a packet of accumulated or xe2x80x9cintegratedxe2x80x9d charge carriers at one TDI stage is sequentially transferred to its neighboring TDI stage in synchronism with movement of an image formed on the photosensitive surface of the TDI image sensor.
Unfortunately, as in the case described previously, presence of the limitation to the coordinate detection resolution can make it difficult, in many cases, to divide the distance on the object plane corresponding to the TDI stage number into the TDI stage number. In addition, the more the TDI stage number, the higher the required pixel size setup accuracy. For example, consider that the TDI stage number is ninety six (96). If the pixel size is increased or decreased by the scale resolution 1 lu which is the minimum variable amount, then the resulting minimum increase/decrease amount in the distance corresponding to such 96 pixels is 96 lu, which makes it impossible, or at least greatly difficult, to provide the intended fine or precise adjustability.
It is therefore an object of the present invention to provide an inspection apparatus for performing visual inspection of an object to be sensed by detecting a 2D image through the main and sub-scanning operations, which apparatus is capable of dividing a desired distance segment along the sub-scanning direction into a desired pixel number.
It is another object of the invention to provide an inspection apparatus for performing visual inspection of a to-be-sensed object by detecting a 2D image through the main and sub-scanning operations, which is capable of setting the pixel size in the sub-scanning direction at an accuracy higher than the scale resolution of an X-Y stage used.
It is still another object of the invention to provide a visual inspection apparatus capable of setting the positional relation of a detection image pattern versus pixels at high accuracy and thus preferably applicable to comparative inspection for repetitive patterns of semiconductor wafers or the like during visual inspection.
In accordance with one aspect of the invention, an image pickup method is provided which picks up a 2D image while causing a projection image of an object of interest as projected onto a linear image sensor to move relative to the linear image sensor in a specific direction (V-scan direction) that is at right angles to the internal scan (H scan) direction of the linear image sensor, and which detects an image while switching the pixel size in the V-scan direction between or among different values during image pickup operations.
In accordance with another aspect of the invention, an image pickup method is provided which picks up a 2D image by permitting a projection image of an object to be sensed that is projected onto a linear image sensor to move relative to the linear image sensor in the V-scan direction perpendicular to the H scanning direction of the linear image sensor, and which detects the 2D image while periodically changing or varying the interval of start pulses for the H scanning.
In accordance with still another aspect of the invention, an image pickup method is arranged to pick up, when a 2D image is picked up while causing a projection image of an object being sensed as projected onto a TDI image sensor to move relative to the TDI linear image sensor, such 2D image of the object is sensed while letting an amount of relative movement vary with time within the total accumulation or xe2x80x9cintegrationxe2x80x9d time periods of the TDI image sensor.
In accordance with yet another aspect of the invention, an image pickup device for use in sensing a 2D image while permitting a projection image of an object of interest as projected onto a linear image sensor to move relative to the linear image sensor in the V-scan direction perpendicular to the H-scan direction of the linear image sensor, the device including position detection means for detecting a position of the object, and pixel size modification means for altering or modifying the setting of the pixel size in the V-scan direction of the linear image sensor on the basis of a position detection signal indicative of the position of the object as detected by the position detection means during image pickup of the linear image sensor.
In accordance with a further aspect of the invention, an image pickup device is provided for picking up a 2D image of an object being sensed by projecting an image of the object onto a linear image sensor while simultaneously letting this projected object image move relative to the linear image sensor in the V-scan direction perpendicular to the H-scan direction of the linear image sensor, the device including position detector means for detecting a position of the object, and start control means for periodically changing or varying the start interval of the H-scanning of the linear image sensor on the basis of a position detection signal representative of the position of the object as detected by the position detector means during image sensing operations.
In accordance with a still further aspect of the invention, an image pickup device is operable to sense a 2D image of an object to be sensed by projecting an image of this object onto a TDI image sensor while at the same time letting this projected object image move relative to the TDI image sensor, the device including position detector means for detecting a position of the object, and control means for varying with time the amount of such relative movement within the total accumulation time period of the TDI image sensor on the basis of a position detection signal indicative of the object position detected by the position detector means during image pickup operations of the TDI image sensor.
In accordance with the principles of the invention, in cases where the target value of pixel size is not an integer times the scale resolution, the pixel size is not set at any fixed value; instead, the H-scan interval greater than the target value (i.e., positive (+) errors) and the H-scan interval less than the target value (i.e., negative (xe2x88x92) errors) are combined together rendering the average pixel size at a divided distance segment L coincident with the target pixel size, thereby enabling subdivision into any desired pixel number. This is done under the condition that certain possible errors corresponding to the scale resolution is acceptable with regard to the size and position of each pixel, except that the divided distant segment L is several times the scale resolution. Note here that the target pixel size value is definable by xe2x80x9cdivided distance segment L÷division pixel number.xe2x80x9d
One preferable way of combining two kinds of H-scan intervalsxe2x80x94i.e. the pixel sizesxe2x80x94is to array the H-scan intervals in a manner which ensures that the average pixel size value within the prescribed given distance segment on the object plane is identical to the target pixel size while at the same time any possible position errors of respective pixels relative to calculation-determinable ideal pixel positions are prevented from accumulating beyond a predefined allowable value. More specifically, the both H-scan intervals, which can have errors of positive (+) and negative (xe2x88x92) polarities with respect to the target pixel size, are arrayed in a way such that resultant accumulated values of positional errors each being caused by one of the H-scan intervals and subsequent the other one do not increase exceeding the allowable value, thereby ensuring that both the sizes and positions of all the pixels fall within a predefined allowable error range.